Maybe it's worth mentioning that Clipboard is just using xclip under the hood (on unix-ish systems). In other words, xclip would need to be
installed for it to work... — which unfortunately often isn't the
case, as xclip doesn't belong to the base tools that come with X11.

In some cases, you might find xsel
installed instead, which is another similar utility.

<off-topic>

BTW, seems like a good occasion to advertise a little trick:
you may use xclip to conveniently copy files between machines
when you already have a working X connection (e.g. ssh login
with X forwarding enabled), and other means like scp or s/ftp
would require you to re-authenticate.

For example, when I log into client sites, I often have to go through a
somewhat cumbersome procedure, requiring me to enter various passphrases,
SecureID tokens and whatnot, until I finally have my SSH/X connection...
(yes I know there's ssh-agent, but it doesn't always help).
In these cases, xclip comes to rescue. I.e. with the following two
aliases in my ~/.bashrc (though it of course also works without aliases):